1. Field of the Invention
The present invention relates to an exposure method, an exposure apparatus and its making method, a device manufacturing method, and a device. More particularly, the present invention relates to an exposure method used to manufacture semiconductor devices and liquid crystal display devices and the like in a lithographic process, an exposure apparatus used in the exposure method and its making method, a device manufacturing method using the exposure method and exposure apparatus, and a device manufactured by the method.
2. Description of the Related Art
Conventionally, in a lithographic process to manufacture devices such as semiconductor devices (CPU, DRAM, and the like), pickup devices (CCD), liquid crystal display devices, and thin film magnetic heads, various exposure apparatus has been used to form a device pattern onto a substrate. In recent years, with the degree of integration of semiconductor devices increasing, the reduction projection exposure apparatus based on the step-and-repeat method (usually referred to as steppers) and a scanning exposure apparatus which is an improvement of the stepper based on a step-and-scan method have become mainstream. These types of apparatus can form a fine pattern onto a substrate such as a wafer or a glass plate (to be generally referred to as a “substrate” or a “wafer” hereinafter) with high throughput.
With these types of projection exposure apparatus, when the amount of exposure light irradiated on the wafer surface (exposure amount) exceeds or is less than the appropriate amount, the line width of the pattern formed may vary. This does not allow a pattern having a desirable line width to be formed, which in turn may lead to the electronic device which is the final product to be a defect. Accordingly, with these types of apparatus, the exposure amount irradiated onto the wafer surface needs to be controlled so that it becomes an appropriate amount.
With the conventional projection exposure apparatus, the relationship between the energy amount of the exposure light which passes through the illumination optical system (hereinafter referred to as “energy amount within the illumination system” for the sake of convenience) and the energy amount of the exposure light which is irradiated on the wafer surface, in other words the image plane (hereinafter referred to as “image plane energy amount” for the sake of convenience) was obtained in advance. And in accordance with the relationship, the conversion factor (or the conversion function) referred to as an α value, to convert the energy amount within the illumination system to the image plane energy amount was determined. Then, on actual exposure, the energy amount within the illumination system was detected by using an optical sensor called an integrator sensor, and based on the detection result and the value (or the conversion function) the image plane energy amount was calculated. And according to the calculation result, exposure amount control of the exposure light irradiated onto each point on the wafer was performed so that the exposure amount was set at an appropriate level.
Meanwhile, the degree of integration of semiconductor devices is becoming higher year by year, and thus demand for exposure apparatus that has a higher resolution is increasing. As a method of improving the resolution of the projection exposure apparatus such as the stepper, increasing the numerical aperture (N. A.) of the projection optical system or shortening the exposure wavelength of the exposure light is typical. However, since increasing the number of numerical apertures limits the depth of focus, the most effective method is considered to be shortening the exposure wavelength.
With the projection exposure apparatus, as the demand for a higher resolution increases, light source which emit light of a shorter wavelength is being used more often as a light source. Recently, as a light source succeeding the krypton fluoride excimer laser (KrF excimer laser), an argon fluoride excimer laser (ArF excimer laser) having an output wavelength of 193 nm has received attention, and using the ArF excimer laser as a light source in an exposure apparatus is now in the stage of practical use. If the argon fluoride excimer laser is used as the exposure light source, it is said that mass production of microdevices having fine patterns with a device rule of 0.18 μm to 0.13 μm would be possible.
When using light of a short wavelength such as the ArF excimer laser as the exposure light, the exposure light being absorbed by atmospheric materials, and the transmittance of the exposure light changing with the elapse of time caused by contaminants adhered to the respective lenses, were considered as problems. Recent researches, though, have confirmed that the transmittance of the optical system such as the projection optical system changed with the elapse of time at a level that cannot be neglected. The so-called cleaning effect can be considered as one of the causes for such a change, and such a cleaning effect is also assumed to have occurred in the case of a KrF excimer laser beam so the properties is clear to some extent. However, in actual, it is difficult to explain the temporal variation of the transmittance described above solely by the cleaning effect. Accordingly, at this point, it is difficult to predict the transmittance variation with high accuracy.
This means, that to obtain the actual amount of light reaching the wafer surface, it is not sufficient enough to only monitor the amount of light (energy amount within the illumination system) using the integrator sensor on the optical path of the illumination system. That is, the conventional method of controlling the exposure amount will most likely fall apart in the near future, due to the fact that the conventional method stood up only on the premise that the transmittance of the optical system (mainly the projection optical system) arranged between the integrator sensor and the image plane (wafer surface) is invariable.
For these reasons, development of a new technology is pressing to control the exposure amount with high precision even if the light source of the exposure apparatus is a light source that emits a light with a wavelength in the vacuum ultraviolet region.
To cope with the transmittance variation described above various attempts are being made. For example, each time the wafer is exchanged, the wafer stage holding the wafer is moved so that an optical sensor called an irradiation amount monitor arranged on the wafer stage comes to a position immediately under the projection optical system. Then, by performing simultaneous measurement with the irradiation amount monitor and the integrator sensor, the transmittance (and the variation) of the optical path on exposure is measured. And, based on the transmittance obtained, the exposure amount is adjusted while correcting the conversion factor described above.
This transmittance measurement, however, cannot be performed unless the wafer stage is moved so that the optical sensor called an irradiation amount monitor comes to a position immediately under the projection optical system, whereas, exposure is performed with the wafer at the position under the projection optical system. Therefore, to perform the simultaneous measurement is time consuming, resulting in a decrease in throughput.
Also, in future, to comply with light of a shorter wavelength, a projection exposure apparatus using a light source emitting the F2 laser beam (wavelength: 157 nm) as the light source will be emerging. Light having this wavelength is greatly absorbed by atmospheric materials. Therefore, when using this light as the exposure light, the optical path needs to be uniformly filled with inert gas to the exposure light, however, it is not that simple to keep the uniformity of the inert gas with the elapse of time. And, the temporal change of the transmittance of the lens coating material within a short period is also a concern. As can be seen, therefore, the temporal change of transmittance of the exposure light needs to be clearer compared to the conventional exposure apparatus. To cope with this situation, it can be assumed that the light amount reaching the image plane (image plane energy amount) needs to be monitored at all times, and not only when the wafer is exchanged. In the conventional method, however, the light amount reaching the image plane could not be measured unless the wafer stage was moved so as to position the irradiation amount monitor immediately under the projection optical system.